Adaptive modulation and coding (AMC) has been used in wireless systems to improve spectral efficiency in a fading environment where signal quality varies significantly. By adjusting the modulation and coding scheme (MCS) in accordance with the varying signal-to-interference-plus-noise ratio (SINR), reliable communication link can be maintained between communicating devices. For example, in CDMA2000 1xEV-DO system, twelve different modulation/coding schemes are provided. AMC is also used in CDMA2000 1xEV-DV and 3GPP HSDPA systems.
To improve performance, in addition to the MCS, other system functions such as channel estimation, transmission power control (TPC), and subchannel configuration can be adjusted in accordance with the state of the communication channel. For example, channel estimation typically utilizes training symbols or pilot data, which are known to both the transmitter and the receiver. For coherent modulation, the channel information can be extracted at the receiver by comparing the pilots and their corresponding received versions. For non-coherent modulation, the received samples of the pilots are used as reference for the detection of the transmitted data.
Channel estimation is an important part of multi-carrier (MC) communication systems such as Orthogonal Frequency Division Multiplexing (OFDM) systems. In conventional OFDM systems, such as IEEE802.11a, 802.11g, 802.16, or DVB-T system, pilots are transmitted for channel estimation. The pilots are fixed and form part of other functions such as MCS, TPC, and subchannel configuration in some wireless systems.
Fast TPC can compensate for fast fading. In a multi-cell multiple-access system, TPC is also used to reduce intra-cell and inter-cell interference and to conserve battery life for the mobile station by transmitting with only necessary power. TPC is one of many functions in some wireless systems, along with MCS, pilot attributes, subchannel configuration, etc.
The subchannel configuration is normally defined and fixed in an operation, and it is usually not considered an adjustable function of the system to be adapted to the user profile and/or operational environment.